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INDEBTEDNESS - Interest Expense (Details) - Convertible Debt - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	May 25, 2018	Mar. 06, 2017
Debt Instrument [Line Items]
Contractual interest expense	$ 5,108	$ 4,023	$ 1,351
Amortization of debt discount and issuance costs	39,139	32,855	14,223
Total	$ 44,247	$ 36,878	$ 15,574
2023 Notes
Debt Instrument [Line Items]
Effective interest rate of the liability component	4.69%			4.69%
2022 Notes
Debt Instrument [Line Items]
Effective interest rate of the liability component	5.34%				5.34%